PositionUnit class abstract

Abstract base class for position units used in element positioning.

PositionUnit defines units that calculate position offsets for elements within a layout container. Unlike size units which determine dimensions, position units specify where elements should be placed along an axis.

Position units can reference various layout properties:

• Fixed pixel offsets
• Viewport and content dimensions
• Scroll positions and overflow amounts
• Child element sizes
• Cross-axis positions

Position units support mathematical operations and can be combined to create complex positioning expressions similar to CSS calc().

Common uses include:

• Absolute positioning of elements
• Sticky positioning relative to scroll
• Centering based on element or viewport size
• Scroll-aware animations

Example:

// Center an element: 50% viewport - 50% child size
final centered = PositionUnit.viewportSize * 0.5 - PositionUnit.childSize() * 0.5;

// Position 20px from viewport end
final offset = PositionUnit.viewportSize - PositionUnit.fixed(20);

Implementers

• PositionCalculated
• PositionChildSize
• PositionConstraint
• PositionContentSize
• PositionCross
• PositionFixed
• PositionOffset
• PositionOverflow
• PositionRelative
• PositionScroll
• PositionUnderflow
• PositionViewportEndBound
• PositionViewportSize

Available extensions

• PositionUnitExtension

Constructors

PositionUnit()

Creates a const position unit.

const

PositionUnit.calc(PositionUnit a, PositionUnit b, CalculationOperation operation)

Creates a calculated position unit combining two position units with an operation.

const

factory

PositionUnit.childSize([Object? key])

Creates a position unit based on a child element's size.

const

factory

PositionUnit.constrained({required PositionUnit position, PositionUnit min, PositionUnit max})

Creates a constrained position unit that clamps values between min and max.

const

factory

PositionUnit.cross(PositionUnit position)

Creates a position unit that uses the cross-axis value of another position unit.

const

factory

PositionUnit.fixed(double value)

Creates a fixed position unit with the specified pixel value.

const

factory

PositionUnit.relative(double factor)

Creates a position unit relative to parent element's size.

const

factory

Properties

hashCode → int

The hash code for this object.

no setter, inherited

runtimeType → Type

A representation of the runtime type of the object.

no setter, inherited

Methods

clamp({PositionUnit min = const PositionFixed(double.negativeInfinity), PositionUnit max = const PositionFixed(double.infinity)}) → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Constrains this position unit within the specified min and max bounds.

computePosition({required ParentLayout parent, required ChildLayout child, required LayoutAxis direction}) → double

Computes the actual position value for this unit.

noSuchMethod(Invocation invocation) → dynamic

Invoked when a nonexistent method or property is accessed.

inherited

times(double other) → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Multiplies a position unit by a scalar.

toCodeString() → String

Converts the position unit to a code string representation.

toString() → String

A string representation of this object.

inherited

Operators

operator *(Object other) → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Multiplies two position units.

operator +(PositionUnit other) → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Adds two position units together.

operator -(PositionUnit other) → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Subtracts one position unit from another.

operator /(PositionUnit other) → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Divides one position unit by another.

operator ==(Object other) → bool

The equality operator.

inherited

operator unary-() → PositionUnit

Available on PositionUnit, provided by the PositionUnitExtension extension

Negates this position unit (equivalent to 0 - this).

Static Methods

lerp(PositionUnit a, PositionUnit b, double t) → PositionUnit

Linearly interpolates between two position units.

Constants

boxOffset → const PositionUnit

A position unit representing the scroll offset (also called boxOffset).

contentOverflow → const PositionUnit

A position unit representing the amount content overflows the viewport.

contentSize → const PositionUnit

A position unit representing the total content size along the axis.

contentUnderflow → const PositionUnit

A position unit representing the amount content underflows the viewport.

scrollOffset → const PositionUnit

A position unit representing the current scroll offset.

viewportEndBound → const PositionUnit

A position unit representing where the viewport ends in content coordinates.

viewportSize → const PositionUnit

A position unit representing the full viewport size along the axis.

viewportStartBound → const PositionUnit

A position unit representing where the viewport starts in content coordinates.

zero → const PositionUnit

A position unit with value zero.